This invention relates to a method and apparatus for mapping magnetic fields such as in permanent or electro-magnets of resistive or superconducting design either alone or in conjunction with gradient coil assemblies associated with such magnets.
Nuclear magnetic resonance (NMR) is the resonance effect of an alternating magnetic field at right angles to a static magnetic field to change the orientation of nuclear magnetic polarization moments within a sample within the static magnetic field. The alternating magnetic field generally is applied as the magnetic component of a radio frequency field which can be applied in a continuous manner over a frequency range or which can be applied as pulses at a fixed frequency. The applied magnetic field causes an induced signal from the sample which signal is uniquely characteristic for a given nucleus and magnetic field strength. In order for the measured induced nuclear magnetism to accurately reflect the characteristic of the nuclear sample, it is essential that the static magnetic field be relatively homogeneous over the sample volume.
The use of currents in coils of varying geometry to establish magnetic field uniformly (homogeneity) is an established practice.
It is also established practise to use ferromagnetic elements inside or outside of the magnet bore in combination with shim coils or alone.
At the present time mapping of the magnetic field in a solenoidal geometry such as in NMR apparatus is conducted either by mechanical movement or NMR imaging. In mechanical movement, a single NMR probe or an array of such probes containing a sample which is small in comparison to the rate of variation of the field over the sample volume but large enough to afford sensible signal to noise, is moved mechanically through the volume of interest within the magnet. Typical trajectories of sample movement have been along the field axis of the void volume within the magnet and about circles in planes perpendicular to the field axis of the magnet. This method requires excess data in that two experiments are required which results in an excessive time to obtain the required data and is mechanically more complicated.
To date utilizing mechanical movement required manual operation and an inordinate time period to effect the mapping and also required a technician be present to record and access each reading.
A second presently utilized method is used in magnetic resonance imaging (MRI) based on pixel-by pixel observation of phase of the signal produced by a large uniform phantom. This technique has been found to be useful down to 0.1 ppm at the low fields used in imaging. However, a large frequency distribution of the signal due to field variation over the volume of interest within the magnet renders this method applicable for fine tuning only. Interpretation ambiguities and instrumental complexities also render this method unattractive for mapping highly inhomogeneous magnets.
Up to the present time, the most accurate and convenient method and apparatus for mapping a static magnetic field is disclosed in U.S. Pat. No. 4,949,044. A liquid sample is mounted on a probe adapted to generate an alternating magnetic field. The alternating magnetic field effects an individual signal from the sample. The sample is moved along a helical path at discrete locations within the static magnetic field and the induced signals at the discrete locations are recorded. As disclosed in this patent, the helical path is formed on a cylindrical surface by virtue of the apparatus disclosed to move the probe. While this method provides substantial advantages over the alternative prior art methods and is significantly simple to effect, the use of a helicylindrical path does not provide the most accurate means for defining the volume of the homogenous magnetic field. This is because the homogeneous magnetic field is most often spherical or ellipsoidal in shape, and the mathematical expression to which the map is fit during analysis involves functions (the spherical harmonics or ellipsoidal harmonics) that are properly orthogonal on the surface of the sphere and not on the surface of a cylinder.
U.S. Pat. No. 4,902,975 discloses a D.C. motor useful for moving capacitor plates in a tomography apparatus (MRI).
Routine mapping applications require several hundred separate measurements at precisely fixed positions. It would be desirable to provide a means for mapping a magnetic field which eliminates the need for a skilled technician to be present. Automatic acquisition requires a computer controlled positioning device that does not perturb the field being measured.